On Saturday morning in Kayl, observers witnessed an unusual weather phenomenon known as a fogbow, or 'white rainbow'. Unlike the vividly coloured rainbow, a fogbow appears as a broad, pale arc in the sky.
A fogbow forms in much the same way as a standard rainbow. The Sun must be behind the observer and shine onto tiny water droplets suspended in fog.
The crucial difference lies in the size of these droplets: In fog, the droplets are much smaller than those in rain.
Because these droplets are so fine, the sunlight's individual colours do not separate as distinctly as they do in a traditional rainbow.
Instead, they blend together, which results in a broad, almost white arc rather than the usual red, orange, yellow, green, and blue stripes.
On photographs, a typical white arc can clearly be seen against the backdrop of the fog layer. Compared to a classical rainbow, a fogbow is much wider, has blurrier edges, and displays little or no colour.
To see a fogbow, the right combination of Sun, fog, and tiny water droplets must come together. Just like an ordinary rainbow, the arc appears on the side of the sky opposite the Sun.
Put differently, a fogbow is indeed a type of rainbow, but the tiny droplets in fog cause it to appear nearly white rather than the familiar spectrum of colours.
Did you capture the 'white rainbow'? If so, feel free to send your images to meteo@rtl.lu.
